Everytime I read about the 51% attack the answer was something like:
is really expensive to get 51% of power on BTC and nobody will invest so much so to destroy the bitcoin.
But I found this site where you can rent hash power it seems to cost only $500K to get 51% for BTC (and much less for the altcoins):https://www.crypto51.app/

Does that mean everybody with just a few dollars can destroy almost any altcoin right now?

EDIT
That site was posted by Charlie Lee (creator of LTC): https://twitter.com/SatoshiLite/status/1001341159405273088 but maybe I'm reading it wrong, becuse the last column says NiceHash-able and for BTC is 2%, LTC 7% and bytecoin 211%, maybe those numbers are the clue but I don't know what it means.
Is that table showing that BTC is hard to attack but other coins like bytecoin are really easy and cheap to attack? maybe someone can clarify this?

1 Answer
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Online mining is that grey area where you don't invest until you're 100% sure. The aforementioned website is likely to scam you if they are providing such a service in the first place.

To address the question, how much will it take to launch a 51% attack;

Firstly you can check the current compute power (hash rate) and it's $ equivalent.

http://rindex.io/attack-cost gives you the exact cost for a 51% attack at any given point of time.

If you compare the calculated hashrate against one of the more popular ASICs on the market, the figure stands at about 2.2 billion. However, this assumes that suddenly half of the network right now goes rogue and decides to attack it. If not, you would need more than 2.2 billion dollars since the hashrate needed has to be 51% of the total hashrate (after the attack starts).

Its rather resource consuming and it isn't that big of a problem since none of the government really cares that much to spend their budget on attacking a cryptocurrency. You probably wouldn't profit all that much when you attack; doubt exchanges credits more than 2 billion dollars worth of Bitcoin automatically without raising a red flag.

Also, If you don't have 51% of the whole network (ie. more hashrate than the others), you don't have a 100% chance of executing an attack. The honest/longest chain still has a chance of overtaking you and you have to acknowledge their blocks. The probability of success decreases with the number of blocks.

Assuming the attacker decides to do 51% attack by purchase lots of ASIC/GPU, but it's not a really accurate number because:

There's no way you could get thousands (maybe millions in the coming future) of ASIC/GPU units easily

Even if they could, it's obvious that the price of ASIC/GPU will be more expensive due to high demand and low supply (this is already the case with graphics cards, poor gamers!)

Unless you have a connection, it's highly unlikely you could use lots of electricity and big mining location without ringing any alarms

Now, what if the attacker wants to hire a pool instead of investing in purchasing the equipment. In the current state where almost all mining power is concentrated within less than ten pools, that's enough to 'buy' a pool admin(s).

Why would even mining pools agree to "rent" their hashpower? If they actually did that, they would quickly lose their users. I don't really think if there would be many farms interested in this form of attack. Long-term earning is much wiser than trying to cheat and sell as much as possible. The rest of the network would quickly realize that the 51% attack was performed. Just look what is going on with Verge which is/was being attacked. The 51% attack is the reason why there are people who think that banning ASICs and mining only with GPUs is a good way to prevent it from happening.

I think I don't know how to read the tables, because the site I've mentioned was posted by Charlie Lee (creator of LTC) so I guess he knows what is talking about: twitter.com/SatoshiLite/status/1001341159405273088 the last column says NiceHash-able and for BTC is 2%, LTC 7% and bytecoin 211%, maybe those numbers are the clue but I don't know what it means. Do you understand it? is that table showing that BTC is hard to attack but other coins like bytecoin are really easy and cheap to attack?
– EnriqueMay 29 '18 at 20:36

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Your question is misleading as it states that this is a website where you can rent hash power to do an attack. As far as reading the tables is concerned, the longer the chain (more users/ blocks/ overall block time/ block-difficulty) the higher is the security, as the requirement for an attack will require as much resource to attack the network. In simple words, an established network will require a hell lot of computing power in comparison to newly formed/ nascent network. Hope this solves your queries.
– Junaid ShaikhMay 29 '18 at 20:41

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@Enrique I believe the last column represents the amount of hashpower nicehash has available to rent, compared to the total hashpower that is mining that coin. So in the case of bytecoin, nicehash's rental capacity has over double the current bytecoin hashrate. For bitcoin, they have 2% (so you could not use Nicehash rentals alone to mount a 51% attack against bitcoin). I suspect Nicehash rentals are a very expensive way to mount an attack in any case.
– chytrikMay 29 '18 at 23:22

@chytrik that makes sense, so that means is possible to destry, BitcoinPrivate for example with only $800?? that does not seem to be expensive at all. Even ETC and BitcoinGold will cost less than $5K
– EnriqueMay 30 '18 at 0:50

@Enrique I would not say it is the cost to ‘destroy’ them, it is just an estimate of the cost of a 51% attack on the network. These numbers are probably more useful as a relative measure of each network’s security than anything else.
– chytrikJun 2 '18 at 17:32